JPS6385021A - Production of single mode optical fiber having low dispersion and loss - Google Patents

Abstract

PURPOSE:To make it possible to produce the titled optical fiber exhibiting longitudinally stabilized transmitting performance and dispersing performance, by deciding a drawing diameter of a core rod according to a specific equation, when the core rod is drawn to a prescribed outside diameter, inserted into a quartz tube and simultaneously drawn. CONSTITUTION:A part to be drawn of a core rod consisting of a core part and clad part provided as surrounded by the core part is measured in a radius direction every prescribed length by a preform analyser to seek a distribution of refractive index. An equivalent core radius ae[mm] of ESI distribution is calculated from the distribution by the prescribed equation. Further, a refractive index n1e and difference of relative refractive index DELTAe[%] of equivalent core is sought by the prescribed equation. A drawing diameter de[mm] of the core rod is calculated using those each value by formula I and II, wherein S is a cross section [mm<2>] of jacket quartz tube, D is an outside diameter [mm] of the core rod, lambdac is a design value [mum] of cut off wavelength and (a) is an outside diameter [mum] of drawn optical fiber.

Description

DETAILED DESCRIPTION OF THE INVENTION [Field of Industrial Application] The present invention relates to a method of manufacturing a single mode optical fiber with low dispersion and low loss.

[Prior art] Single-mode optical fibers are widely used as optical transmission lines in optical communication technology, optical measurement technology, etc., but these optical fibers mainly have a cladding with a low refractive index provided around the outer periphery of the core. It has a structure in which a jacket is provided around the outer periphery of the cladding. Such optical fibers generally consist of a core rod consisting of a core part and a cladding part, which is stretched to a predetermined diameter, and then inserted into a jacketed quartz tube.
Manufactured by simultaneous drawing.

Here, the conventional method of determining the drawn diameter of the core rod will be described below.

First, the area to be drawn of the manufactured core rod was measured at several locations using a preform analyzer, and the average value of the outer diameter D[s
], average value of core diameter (half width of refractive index distribution) 2T [m]
and the average value Δ[%] of the relative refractive index difference.

Next, the stretched diameter d of the core rod is determined according to the following equation (1) + 2), with the target cutoff wavelength set as λC.

However, S is the nominal cross-sectional area of the jacketed quartz tube used [
jII2], So is the design cross-sectional area [2] for setting the cutoff wavelength to λC, and no is the average value of the refractive index of the core. In addition, γ is a correction coefficient, which was introduced because the actual profile does not have a perfect step shape.
The designer looks at the profile and makes a decision.

[Problems to be Solved by the Invention] However, the following problems occur in such a method of determining the extended core rod diameter d.

(2) Since the correction coefficient γ is determined by the designer's judgment, individual differences occur in the cutoff wavelength λC of the resulting fiber.

(2) Since the average value of measured values at several locations is used as the outer diameter, core diameter, and relative refractive index difference of the core rod, there is a risk that there will be parts of the optical fiber that do not meet the specifications. As a result, the transmission characteristics of the optical fiber may not be uniform in the longitudinal direction, and the loss may not be low.

■ If there are irregularities such as angular shape or skirting in the refractive index distribution of the core rod, it will be difficult to find the optimal value for the correction coefficient γ, and the resulting cutoff wavelength may differ significantly from the target value.

(2) Since the correction coefficient γ is required, the accuracy of the cutoff wavelength λC obtained is at most ±0.06 jH4.

Therefore, even if an attempt is made to obtain a low dispersion fiber, the dispersion characteristics are determined by the cutoff wavelength when the relative refractive index difference is constant, resulting in poor dispersion controllability and poor drawing yield.

SUMMARY OF THE INVENTION It is therefore an object of the present invention to provide a manufacturing method capable of solving the above-mentioned problems of the prior art and producing a low-dispersion, low-loss single-mode optical fiber that exhibits stable transmission and dispersion characteristics in the longitudinal direction. .

[Means for Solving the Problems] In order to achieve the above object, the method for manufacturing a low-dispersion, low-loss single mode optical fiber of the present invention stretches a core rod to a predetermined outer diameter and inserts it into a jacketed quartz tube. After insertion, the method for manufacturing a single mode optical fiber by crushing and drawing the quartz tube so that it is integrated with a core rod,
A value determined by the following formula using the elongated diameter de[ms+1] of the core rod, the refractive index n1e of the core in an equivalent step-type refractive index distribution, the relative refractive index difference Δe [%], and the equivalent core radius ae[ai+] This is the method to do so.

(However, S is the cross-sectional area of the jacket quartz tube [Ia2], D is the outer diameter of the core rod [diameter], λC is the cut-off wavelength design value [*],
a is the outer diameter [*] of the drawn optical fiber) [Function] The present invention provides an equivalent step-type refractive index distribution of the core rod, that is, E S I (E Quivalent S t
Since the elongated diameter de of the core rod is determined based on the distribution (epI nde can.

Specifically, the actual refractive index distribution of the core rod is measured and replaced with an equivalent step-type refractive index distribution, and the refractive index n18 of the core in this step-type refractive index distribution is determined. The extended diameter de of the core rod is determined using the relative refractive index difference Δe, the equivalent core radius ae, the cross-sectional area S of the jacket quartz tube, etc.

Furthermore, if the cross-sectional area S of the jacketed quartz tube is measured as a function of its longitudinal direction by a projection method, etc., and this measured value is used to determine the extended diameter de of the core rod, then the transmission characteristics and dispersion characteristics in the longitudinal direction A single mode optical fiber with excellent stability can be obtained.

[Example] Embodiments of the present invention will be described below with reference to the accompanying drawings.

First, the portion of the core rod that is to be drawn, which consists of a core portion and a cladding portion provided to surround it, is measured in the radial direction every 1jII+ to 2 m in length using a preform analyzer to obtain the refractive index distribution n(r). demand.

Next, from this refractive index distribution n (r), an equivalent core radius ae [ml of the ESI distribution expressed by the following formula is calculated.

However, rN -n+ nl : Maximum refractive index of the core part n2 : Refractive index of the cladding part Furthermore, based on the following formula, the equivalent refractive index of the core 1e Here, the actual refractive index distribution 1 of the core rod is shown in Fig. 1. and E
The relationship of SI81 distribution is shown. The ESI81 distribution is a complete step type distribution.

Roughly speaking, the equivalent core radius ae [ ], the refractive index n1e and the relative refractive index difference Δe [
%], the stretched diameter de[ml] of the core rod is calculated as follows.

However, S is the cross-sectional area of the jacket quartz tube [aI2], D is the outer diameter of the core rod [H], and λC is the cut-off wavelength design value [
4], a is the outer diameter [*] of the drawn optical fiber.

In this way, the stretched diameter da at each measurement point is determined, and the core rod is stretched based on the results. Then, this core rod is inserted into a jacketed quartz tube and simultaneously drawn to produce a single mode optical fiber.

When the outer diameter a of the drawn optical fiber was set to 1257J, a plurality of single mode optical fibers with various cutoff wavelength settings λC were created, and the cutoff wavelengths of each were actually measured, and the results shown in Figure 2 were obtained. It was done. That is,
It was possible to control the cutoff wavelength with an accuracy within ±0.015 tones, and the transmission and dispersion characteristics were extremely good and uniform in the longitudinal direction.

Note that the cross-sectional area S of the jacketed quartz tube used may vary in the longitudinal direction, so for example,
It is effective to measure the cross-sectional area S of the jacketed quartz tube as a function of its longitudinal direction using the projection method shown above.

That is, the jacketed quartz tube 3 is rotated around its axis while the jacketed quartz tube 3 is irradiated with light from the tungsten halogen lamp 4 through the lens 5. Then, the jacketed quartz tube 3 is imaged from the side with the TV camera 6, and while the image is projected onto the monitor TV 8 via the camera controller 7, the variation in the cross-sectional area S of the jacketed quartz tube 3 is measured by the control section 9.

By storing the function of the cross-sectional area S of the jacketed quartz tube 3 determined in this way in an automatic stretching device and controlling the stretching speed by a program, it is possible to achieve uniform transmission and dispersion characteristics in the longitudinal direction of the layer. Optical fiber is obtained.

[Effects of the Invention] As explained above, according to the present invention, the following excellent effects are exhibited.

(1) Since the cutoff wavelength can be set to IIIn with high precision, a low dispersion, low loss single mode optical fiber with excellent stability of transmission characteristics and dispersion characteristics in the longitudinal direction can be obtained.

■ Even if there is a disturbance in the refractive index distribution of the core rod, the cutoff wavelength can be controlled with good reproducibility regardless of individual differences, improving the drawing yield.

(3) Therefore, cost reduction of single mode optical fiber is achieved.

[Brief explanation of the drawing]

Figure 1 is an explanatory diagram showing the relationship between the actual refractive index distribution and the ES ■ distribution of the core rod, and Figure 2 is the relationship between the designed value and the actual measured value of the cutoff wavelength of the optical fiber manufactured by the method of the present invention. FIG. 3 is a schematic diagram of an apparatus for measuring variations in cross-sectional area of jacketed quartz tubes. In the figure, 1 is the refractive index distribution of the core rod, and 2 is the ES■ distribution.

Claims (3)

[Claims] (1) A method of manufacturing a single mode optical fiber by stretching a core rod to a predetermined outer diameter and inserting it into a jacketed quartz tube, and then drawing the quartz tube while crushing it so that it becomes integrated with the core rod, The stretched diameter de [mm] of the core rod is determined by the following formula using the refractive index n_1_e of the core in an equivalent step-type refractive index distribution, the relative refractive index difference Δe [%], and the equivalent core radius ae [mm]. A method for manufacturing a low-dispersion, low-loss single-mode optical fiber characterized by a low-dispersion, low-loss single-mode optical fiber. de=D√(S/S_0_e) (However, S_0_e=π/4[([2n_1_eπaae√{2
△e/100}/[2・2.405λc])^2-D
^2] S is the cross-sectional area of the jacket quartz tube [mm^2], D
is the core rod outer diameter [mm], λc is the cutoff wavelength design value [μm], a is the outer diameter of the drawn optical fiber [μm]
) (2) A manufacturing method according to claim 1, characterized in that the cross-sectional area S of the jacketed quartz tube is measured as a function of its longitudinal direction. (3) The manufacturing method according to claim 2, wherein the cross-sectional area S of the jacketed quartz tube is measured by a projection method.